Oncogenic activation of MAPK in thyroid cells leads to loss of expression of genes required for thyroid hormone biosynthesis, including the sodium iodide transporter (NIS) and thyroid peroxidase (TPO). Tumors with BRAF mutation have lower expression of NIS, which likely explains why BRAF mutant PTCs are often resistant to RAI therapy. We developed mouse models of thyroid cancer driven by BRAF-V600E, and these tumors also lose the ability to concentrate radioiodine, which is restored by treatment with RAF or MEK inhibitors. Moreover, the MEK inhibitor AZD6244 reactivated iodide uptake at metastatic sites in patients with RAI- refractory thyroid cancer, allowing many of them to be treated with 131-iodine, with remarkable clinical responses. These beneficial results were seen although MEK inhibitors do not fully block MAPK signaling in thyroid cancer cells, because they relieve a feedback leading to upregulation of receptor tyrosine kinases, in particular HER3, which confers resistance to therapy. In addition, activation of TGF signaling, which is a common feature of advanced forms of thyroid cancer, may be further induced in response to MAPK inhibitors, leading to further downregulation of NIS. The goals of this project are to determine how to optimize inhibition of MAPK signaling to further enhance radioactive iodine uptake and response to RAI therapy in thyroid cancer. This will be done through the following specific aims: 1) Determine the effect of MEK inhibitors on the kinetics of iodine-124 incorporation in patients with metastatic RAI refractory thyroid cancer, and test the hypothesis that this is due to increased expression of genes required for incorporation of inorganic iodide into proteins. 2) Determine if a combination of inhibitors that target MAPK and HER3 signaling is more effective in restoring RAI incorporation than the single agents in mouse models of BRAF-induced thyroid cancers. 3) Determine if pharmacological inhibitors of TGF signaling enhance iodide uptake alone or in combination with inhibitors of the RAF-MEK-ERK pathway.4) Evaluate the response to 131I therapy of murine thyroid cancers pretreated with the combination therapy/s showing the best effects on 124I dosimetry.